The uptake of atmospheric oxidized organics on acid clusters is relevant for atmospheric new particle formation. We investigate the pickup of methanol (CH3OH) on mixed nitric acid-water clusters (HNO3)M(H2O)N by a combination of mass spectrometry and cluster velocity measurements in a molecular beam. The mass spectra of the mixed clusters exhibit (HNO3)m(H2O)nH+ series with m = 0-3 and n = 0-12. In addition, CH3OH·(HNO3)m(H2O)nH+ series with very similar patterns appear in the spectra after the methanol pickup. The velocity measurements prove that the undoped (HNO3)m(H2O)nH+ mass peaks in the pickup spectra originate from the neutral (HNO3)M(H2O)N clusters which have not picked up any CH3OH molecule, i.e., methanol has not evaporated upon the ionization. Thus the fraction of the doped clusters can be determined and the mean pickup cross section can be estimated, yielding σs¯≈ 20 Å2. This is compared to the lower estimate of the mean geometrical cross section σg¯≈ 60 Å2 obtained from the theoretical cluster geometries. Thus the "size" of the cluster corresponding to the methanol pickup is at least 3-times smaller than its geometrical size. We have introduced a method which can yield the absolute pickup cross sections relevant to the generation and growth of atmospheric aerosols, as illustrated in the example of methanol and nitric acid clusters.

Electron attachment to mixed HNO3/H2O clusters yields several atmospherically relevant species such as NO3 –, HONO and OH radical.

When ionizing radiation passes biological matter, a large number of secondary electrons with very low energies (<3 eV) is produced. It is known that such electrons cause an efficient fragmentation of isolated nucleobases via dissociative electron attachment. We present an experimental study of the electron attachment to microhydrated nucleobases. Our novel approach allows significant control over the hydration of molecules studied in the molecular beam. We directly show for the first time that the presence of a few water molecules suppresses the dissociative channel and leads exclusively to formation of intact molecular and hydrated anions. The suppression of fragmentation is ascribed to caging-like effects and fast energy transfer to the solvent. This is in contrast with theoretical prediction that microhydration strongly enhances the fragmentation of nucleobases. The current observation impacts mechanisms of reductive DNA strand breaks proposed to date on the basis of gas-phase experiments.

We present cross sections for pickup of several atmospherically relevant molecules on ice nanoparticles with the 0.5-3 rim diameter range. The experimental values are supported by molecular dynamics simulations and analytical calculations based on long-range cluster-molecule potentials. The cross sections are all considerably larger than the geometrical cross section of nanoparticle and vary significantly for different guest molecules.

We report cross sections for pickup of guest molecules on neutral argon and water clusters with the mean sizes in the range from N = 50 to 600. The experiments are supported by molecular dynamics simulations and analytical models based on the interaction potentials. The cross sections for argon clusters are consistent with their assumed spherical shape and follow approximately the theoretically justified N(1/3) dependence. On the other hand, the cross sections of water clusters depart from this dependence and are considerably larger starting from N ≥ 300. We interpret this increase of cross section by the occurrence of highly irregular shapes of water clusters produced in the supersonic expansion of water vapor under the conditions of the large cluster generation.

The photodissociation dynamics of HX (X = Cl, Br) molecules deposited on large ArN and (H2O)N, N̄ ≈ 10(2)-10(3), clusters is investigated at 193 nm using velocity map imaging of H and Cl photofragments. In addition, time-of-flight mass spectrometry after electron ionization complemented by pickup cross section measurements provide information about the composition and structure of the clusters. The hydrogen halides coagulate efficiently to generate smaller (HX)n clusters on ArN upon multiple pickup conditions. This implies a high mobility of HX molecules on argon. On the other hand, the molecules remain isolated on (H2O)N. The photodissociation on ArN leads to strong H-fragment caging manifested by the fragment intensity peaking sharply at zero kinetic energy. Some of the Cl-fragments from HCl photodissociation on ArN are also caged, while some of the fragments escape the cluster directly without losing their kinetic energy. The images of H-fragments from HX on (H2O)N also exhibit a strong central intensity, however, with a different kinetic energy distribution which originates from different processes: the HX acidic dissociation followed by H3O neutral hydronium radical formation after the UV excitation, and the slow H-fragments stem from subsequent decay of the H3O. The corresponding Cl-cofragment from the photoexcitation of the HCl·(H2O)N is trapped in the ice nanoparticle.

The chlorofluorocarbons (CFCs) are extremely inert compounds which decay only in the upper stratosphere after absorption of high energy solar radiation. The decomposition pathway can lead to formation of reactive Cl radicals which triggers the ozone depletion cycle. One of the crucial questions for stratospheric chemistry is how is the photodissociation dynamics of these species influenced by the aerosol particles in the stratosphere? The polar stratospheric cloud (PSC) particles can adsorb CFCs and change their photochemistry. To investigate this effect, we have studied the UV photodissociation of CF2Cl2 (CFC-12) molecule in molecular beam experiment: first we study the photodissociation of an isolated molecule, then simulate the influence of the solvent by the simplest archetype solvent species, i.e. rare gas atoms in Ar and Xe clusters, and finally we will investigate the CFC photodissociation on ice nanoparticles. We have overlapped the molecular beam with two laser beams. The photodissociation was studied at 193 nm, and Cl(P3/2) and Cl*(P1/2) fragments were ionized using (2+1) REMPI schemes around 235 nm. The ionic products were guided onto position sensitive 2D detector by velocity mapping electrode configuration. From the images recorded by a CCD camera complete 3D information about the fragment velocities can be obtained. The Cl-fragments images of are shown in Fig. 1: (a) corresponds to the photodissociation of isolated molecule, and (b) originates from CF2Cl2 in clusters. The circular edges in both images correspond to the fast Cl fragment, while the central bright spot in (b) corresponds to Cl atoms with near zero kinetic energy due to the cluster caging effect. The graph below the images shows the corresponding Cl-fragment kinetic energy distributions (KEDs). These results resolve a discrepancy from the previous CF2Cl2 molecule photodissociation studies (Baum 1993, and Yen 1993). The presence of the slow Cl fragments in the spectra points to the secondary dissociation of CF2Cl fragment radical. Our measurements show that more than one Cl atom can be released per one UV-photon from a CF2Cl2 molecule, and also that a Cl2 molecule can be released instead. In addition all these fragments can be trapped by cage effect if the photodissociation happens on PSC particles. All these conclusions can have significant consequences for stratospheric ozone modelling. (b) (a)

A unique versatile molecular beam CLUB (CLUstr Beam) apparatus has recently been exploited for various studies of atmospherically relevant aerosol particles in vacuum. These studies will be briefly reviewed demonstrating the significant potential which the implementation of the molecular beam techniques has for the aerosol studies. In molecular beams clusters are prepared by expansion of suitable gases into the vacuum through supersonic nozzles. Depending on the expansion conditions the clusters of various sizes can be generated ranging from a few molecules to small aerosol particles with sizes in the nanometer range composed of hundreds molecules. Our CLUB apparatus enables various experiments with these particles: (1) doping them with different molecules; (2) investigating photodissociation and photochemistry in/on the clusters by the fragment velocity imaging method; (3) studying the composition, dynamics and reactions by a unique reflectron time-offlight mass spectrometer. Several experiments will illustrate the variety of information about the small aerosol particles gained with the CLUB apparatus. First, we will exemplify the pickup cross sections of small ice nanoparticles for uptake of various atmospheric molecules (Lengyel, Kočišek et al, 2012). The cross sections were measured by precise velocity measurements of particles after their passage through a pickup cell filled with the particular gas. The cluster velocity change in dependence on the pickup pressure allows the pickup cross section determination. The experimental results are in good agreement with molecular dynamics simulations. They show that the actual cross section for the pickup is significantly larger than the geometrical cross section. This effect should be considered in the atmospheric models and nucleation theories. Photochemistry of ice nanoparticles doped by hydrogen halide molecules HX (X= Cl, Br, I) is highly relevant to the stratospheric ozone depletion process. We have studied HX(H2O)n, n= 10 2 -10 3 , clusters under the influence of UV laser radiation at 193 nm and 243 nm. A series of experiments and theoretical calculations (Poterya, 2007) (Poterya, 2011) (Ončák, 2011) revealed that the HX molecule is acidically dissociated on the ice particle and a neutral hydronium radical H3O is generated upon the UV excitation. This radical also represents a cluster model for solvated electron. Most recently we have also investigated the photodissociation of CF2Cl2 molecule in clusters revealing the detailed dynamics of the molecular dissociation, and Cl-fragment caging and escape from the clusters. Finally, we have studied mixed nitric acid/water ice nanoparticles in two different mass spectrometric experiments (Lengyel, Pysanenko et al, 2012). The comparison between the electron ionization and photoionization after Na-pickup showed that HNO3 molecules act as very effective condensation nuclei, and no pure water clusters were generated even from vapors with 100 fold excess of water molecules. Besides, we have demonstrated that the Na doping can be used for detection of species reacting with Na or solvated electron in the aerosols. In general, all these results are relevant for the ozone depletion process in the stratosphere.

Uptake of several atmospheric molecules on free ice nanoparticles was investigated. Typical examples were chosen: water, methane, NO(x) species (NO, NO(2)), hydrogen halides (HCl, HBr), and volatile organic compounds (CH(3)OH, CH(3)CH(2)OH). The cross sections for pickup of these molecules on ice nanoparticles (H(2)O)(N) with the mean size of N≈260 (diameter ~2.3 nm) were measured in a molecular beam experiment. These cross sections were determined from the cluster beam velocity decrease due to the momentum transfer during the pickup process. For water molecules molecular dynamics simulations were performed to learn the details of the pickup process. The experimental results for water are in good agreement with the simulations. The pickup cross sections of ice particles of several nanometers in diameter can be more than 3 times larger than the geometrical cross sections of these particles. This can have significant consequences in modelling of atmospheric ice nanoparticles, e.g., their growth.

We have implemented the velocity map imaging technique to study clustering in the pulsed supersonic expansions of hydrogen bromide in helium, argon, and xenon. The expansions are characterized by direct imaging of the beam velocity distributions. We have investigated the cluster generation by means of UV photodissociation and photoionization of HBr molecules. Two distinct features appear in the hydrogen atom photofragment images in the clustering regime: (i) photofragments with near zero kinetic energies and (ii) "hot" photofragments originating from vibrationally excited HBr molecules. The origin of both features is attributed to the fragment caging by the cluster. We discuss the nature of the formed clusters based on the change of the photofragment images with the expansion parameters and on the photoionization mass spectra and conclude that single HBr molecule encompassed with rare gas "snowball" is consistent with the experimental observations.

Pickup of several molecules, H(2)O, HBr, and CH(3)OH, and Ar atoms on free Ar(N) clusters has been investigated in a molecular beam experiment. The pickup cross sections of the clusters with known mean sizes, Ñ≈ 150 and 260 were measured by two independent methods: (i) the cluster beam velocity decrease due to the momentum transfer of the picked up molecules to the clusters, and (ii) Poisson distribution of a selected cluster fragment ion as a function of the pickup pressure. In addition, the pickup cross sections were calculated using molecular dynamics and Monte Carlo simulations. The simulations support the results of the velocity measurements. On the other hand, the Poisson distributions yield significantly smaller cross sections, inconsistent with the known Ar(N) cluster sizes. These results are discussed in terms of: (i) an incomplete coagulation of guest molecules on the argon clusters when two or more molecules are picked up; and (ii) the fragmentation pattern of the embedded molecules and their clusters upon ionization on the Ar cluster. We conclude that the Poisson distribution method has to be cautiously examined, if conclusions should be drawn about the cluster cross section, or the mean cluster size Ñ, and the number of picked up molecules.

We have carried out a selected ion flow tube mass spectrometry (SIFT-MS) study of the concentrations of the sulfur-containing compounds H(2)S (using H(3)O(+) precursor ions), CH(3)SH (H(3)O(+)), (CH(3))(2)S (O(2)(+)), (CH(3))(2)S(2) (NO(+)) and CS(2) (O(2)(+)) in single exhalations of mouth-exhaled breath and nose-exhaled breath and in the static gas in the oral cavity for two healthy volunteers. The primary purpose of the study was to show how compounds present in breath at levels as low as a part per billion (ppb) can be identified and quantified if the overlap of 'impurity' isobaric ions with the analytical product ions for each trace compound is identified and accounted for. The H(2)S measurements are straightforward using H(3)O(+) precursor ions, since no overlapping ions are recognized and its breath concentration is relatively high at typically 20-70 ppb. Thus, its concentration distribution for two healthy volunteers has been obtained over a period of a few weeks. The situation is very similar for CH(3)SH, but to analyse this compound we had to study the kinetics of its reactions with the SIFT-MS reagent ions H(3)O(+), NO(+) and O(2)(+) in order to provide the required kinetics library data for this compound. It is seen that CH(3)SH, (CH(3))(2)S and (CH(3))(2)S(2) are present in the mouth breath/cavity at lower levels of <10 ppb. The measurements of the levels of H(2)S and these compounds in the nose-exhaled breath and the closed mouth indicate that they are largely produced in the oral cavity, although there is some indication that (CH(3))(2)S is partially systemic in these two volunteers. It was not possible to quantify CS(2) in the breath because of serious interference (overlapping ions) due to the presence of carbon dioxide and acetone that inevitably occur in exhaled breath. This study paves the way for the accurate analysis of these sulfur compounds in halitosis and potentially for probing the diseased state, especially liver disease, by breath analysis. To demonstrate the simplicity of measuring these compounds when they are present at levels of about 100 ppb and greater, data are presented on the emissions of these sulfur-containing compounds from Pseudomonas bacterial cultures in vitro.

The recombination of D-3(+) and D-5(+) ions with electrons in a He-Ar-D-2 flowing afterglow plasma is reported. Low temperature (T = 130-300 K) and high pressure of the He buffer gas (900-1200 Pa) was used to enhance the formation of D-5(+) ions in the afterglow plasma. The deuterium partial number density was varied over a large range ([D-2] = 1 x 10(12)-3 x 10(15) cm(-3)) to study its influence on plasma decay. At low [D-2], D-3(+) ions dominate in the afterglow and the plasma decay is controlled by the recombination of D-3(+) ions (rate coefficient alpha(3)). At high [D-2] and lower temperatures, D-5(+) ions are effectively formed and the plasma decay is controlled by the recombination of D-5(+) ions (alpha(5)). In the intermediate region, the rate of recombination is given by the partial densities of both ions, D-3(+) and D-5(+). These partial densities are influenced by conditions in the plasma and they are controlled by ion-molecule reactions (D-3(+) + D-2 <-> D-5(+). If the ion-molecule reactions are fast in comparison with the recombination processes the plasma decay can be characterized by the equilibrium constant KC and by recombination rate coefficients alpha(3) and alpha(5). By monitoring of the plasma decay, all three constants alpha(3) (190 K) = (1.4 +/- 0.5) x 10(-7) cm(3) s(-1), alpha(5) (190 K) = (3 +/- 1) x 10(-6) cm(3) s(-1) and K-C (190 K) = (3.8 +/- 2.0) x 10(-16) cm(3) are determined.

Collisions of cations and dications C7H8+/2+, C7H7+/2+, and C7H62+ generated by electron ionization of toluene with a highly oriented pyrolytic graphite surface were investigated in scattering experiments at the incident energy of 25.3eV, incident angle of 60° (with respect to the surface normal) and at surface temperatures of 300 and 900K. The survival probability of ions was rather large, about 10% for the cations and about twice as large for the dications. Only singly-charged ions were observed in the mass spectra of product ions for both singly- and doubly-charged incident ions. In agreement with earlier conclusion of Cooks et al., the primary process in surface collisions of the dications is a single-charge exchange between the approaching dication and the surface at larger distances; hence, the mass spectrum of product ions in fact results from surface interactions of internally excited monocations. This scenario is also supported by measured translational energy distributions and angular distributions of the major product ions which are very similar for both dication- and cation-collisions. Two mechanisms of formation for the fragment ions observed are suggested: either via unimolecular decomposition of the inelastically scattered projectile ion or via decay of the protonated projectile formed by endoergic hydrogen transfer from the surface hydrocarbons to the projectile ion. The translational energy distributions of ions originating from dissociation of the surface-excited projectile ions peak at higher energies than those of the ions resulting from decomposition of surface-protonated precursor ions.

The recombination of H+3 ions with electrons has been studied in afterglow plasma in three different experiments. In two experiments, using the Variable Temperature Stationary Afterglow (VT-AISA) and the Variable Temperature Flowing Afterglow (VT-FALP) techniques, a decay of the electron number density was measured by an electrostatic Langmuir probe to determine the recombination rate coefficient. In the third experiment a near infrared Cavity Ring-Down Absorption Spectrometer (CRDS) was used to monitor the decay of the H+3 (v = 0) ion density during the afterglow. Measurements were carried out in helium buffer gas with small admixtures of argon and hydrogen at total pressures ranging from 150 up to 1200 Pa and at buffer gas temperatures ranging from 100 up to 330 K. In the experiments the partial number density of hydrogen was varied from 5 × 1010 up to 1 × 1016 cm−3 and for this broad range of hydrogen number densities effective recombination rate coefficients were obtained, which varied over three orders of magnitude from 2 × 10−9 cm3s−1 at [H2] = 5 × 1010 cm−3 up to 3 × 10−6 cm3s−1 at [H2] = 1 × 1016 cm−3. Using our experimental results we discuss possible mechanisms of recombination in hydrogen plasma in a very broad range of several parameters: buffer gas pressure, temperature, electron number density, hydrogen number density and internal excitation of recombining ions.

We report a study of the recombination of H3+ (v = 0) ions with thermal electrons at 330 and 100 K. A near infrared cavity ring-down absorption spectrometer (CRDS) working on the v2 = 3 ← 0 transition of H3+ (λ = 1382 nm) has been used to monitor the H3+ (v = 0) ion number density in decaying afterglow plasma. The plasma was created in helium gas with small admixtures of argon and hydrogen by pulses of microwaves. The measurements were carried out for hydrogen number densities ranging from 1013 up to 1016 cm−3. The total pressure in the discharge tube was 4–10 mbar. The temperature of the recombining ions was determined from the Doppler broadening of absorption lines. The obtained effective recombination rate coefficients are α (H3+ (v = 0)) = (0.8 ± 0.3) × 10−7 cm3s−1 and α(H3+ (v = 0)) = (2.3 ± 1.1) × 10−7 cm3s−1 at 330 and 100 K, respectively. The influence of the formation of H5+ at higher pressures and lower temperature is also discussed.

Decay studies were carried out in the afterglow of low temperature plasma generated by pulsed microwave discharge in He with small admixture of Ar and H-2. In such a mixture all ions formed during a microwave discharge are by ion-molecule reactions converted to H-3(+) ions. The decay of the H-3(+) dominated afterglow plasma was monitored by means of infrared cavity ring-down spectrometer (CRDS) employing a cw-diode laser. Measurements were carried out at temperature of similar to 350 K and total pressure of 0.66-2.66 kPa. The CRDS signal on the nu(2)= 3 <-- 0 transition of H-3(+) (observed at around 1.4 mum) was detected at different moments during the discharge afterglow. Knowing the absorption cross-section the evolution of the absolute number density of H-3(+) (nu=0) during the afterglow was determined. The recombination rate coefficient calculated from the decay curves (at hydrogen number density of [H-2] = 3-8x10(20) m(-3)) is alpha = (1.8+/-0.8)x10(13) m(3) s(-1). The absorption spectra provided us the kinetic temperature of H-3(+) (nu=0) ions during discharge and the afterglow. Detailed description of the experimental set up is also given here.

The recombination of H 3 + ions with electrons has been studied in pulsed afterglow plasma. Measurements were carried out in helium buffer gas with small admixtures of argon and hydrogen. Molecular hydrogen number density was varied from 1011 up to 3×1013 cm−3. The gas temperature was varied from 130 K up to 250 K, and the total pressure ranged from 150 Pa to 320 Pa. Evolution of electron number density during afterglow was monitored by Langmuir probe. Measurements were carried out using new Variable Temperature Stationary Afterglow apparatus (VT-AISA). From the electron number density decays the recombination rate coefficient (α) was determined. Temperature dependence of recombination rate coefficient over broad range of hydrogen number densities was obtained from measured data. We observed strong dependence of α on partial pressure of hydrogen and on temperature. The measurement is continuation of our previous studies, which were carried out at 260 K only.

We attempted to compare several probe theories that described collection of charged particles by Langmuir probe at elevated pressures. We used stationary afterglow environment with well-defined ionic composition to measure the electron density as a function of time. Afterglow plasma has the advantage that probability of secondary electron emission from probe surface when the probe works in positive ion acceleration regime is negligible and hence the probe data are more dependable. Since estimation of the rate of recombination required knowledge of absolute values of plasma density, we were able, in case of the ions the rate of recombination of which was known, to calculate the electron density, which corresponded to the known rate of recombination. Consequently we were able to compare apparent electron density calculated from the probe data using different theories with the “reference” electron density that we obtained from the known rate of recombination. Up to several mbar pressure the differences between the “reference” value of electron density and value calculated using the collisionless theories of electron collection by Langmuir probe stay within reasonable error limits.

The recombination of spectroscopically identified H3+(v=0) ions with thermal electrons has been studied in pulsed afterglow plasma by means of an infrared cavity ring-down spectrometer (CRDS). Time-resolved measurements of the H3+(v=0) density were carried out in helium buffer gas with small admixtures of argon and hydrogen. The gas temperature was ∼330K, and the total pressure ranged from 8 to 16mbar. The CRDS signal on the ν2=3←0 transition of H3+ (λ=1.4μm) was monitored as a function of time during the discharge afterglow. Since the absorption cross-section is known, the decay of the H3+(v=0) number density and hence, the recombination coefficient can be deduced. At hydrogen number densities [H2]=1×1014 to 8×1014cm−3 the measured recombination rate coefficient was found to be α=(1.6±0.6)×10−7cm3s−1.